The combination of an ultrasonic wave generator and curved or planar reflector is well known in the art as disclosed, for example, in Rosenberg et al. U.S. Pat. No. 2,480,199 wherein a pencil beam from a supersonic generator is reflected off a reflector designed to produce a radiation pattern simulating a particular radio object locating device. In Rosenberg et. al., the reflector convexly and concavely curved in two mutually perpendicular directions such that the radiated pattern curves to a focus at approximately the middle of the range to be covered to achieve a narrower beam width without the reflector. Bacon patent 3,028,752 discloses an ultrasonic testing apparatus in which the combination of a sonic generator-reflector produces a focused beam with a long liquid coupling gap and suggests using an electromechanical transducer for producing a beam of ultrasonic energy and a curbed reflector wherein the curved portion of the reflector may be a section of a sphere, cylinder, parabaloid of the revolution, section of a right circular cone with the time required for the beam to cover the length of the liquid couplet path being greater than the time required for the entire path within the solid object to be traverse by the beam. In Bouyoueces U.S. Pat. No. 3,532,182, a hydroaccoustic impulse generator comprises the reflector shaped as a pseudosphere converting impulse signals to a omnidirectional pattern. Finally, in Hurwitz U.S. Pat. No. 3,965,455 a compound reflector focuses a sonic beam to obtain a line of focus. These sonic generator-reflector systems are complex and/or expensive and do not satisfy or solve the problem of providing low cost beam expanders for narrow beam electrostatic transducers for ultrasonic ranging, guidance and surveillance systems which are efficient and easy to produce and do not have significant alignment problems. According to the present invention, a low cost electrostatic planer transducer element, such as a Polaroid electrostatic transducer, generates compression and rarefaction waves which are essentially perpendicular to its planar surface and hence the beam is relatively narrow. The Polaroid transducer produces a conical ultrasonic beam at 50 kHz that is approximately 10 degrees wide. The object of the present invention is to provide a low cost beam transformer and method for such a transmitted wave and to couple the weaker return pulse energy to the transducer for detection. For this purpose, a beam transformer is combined with the planar electrostatic transducer for converting the narrow parallel beam of such a transducer to a beam which is broad along one axis and narrow along a transverse axis. The beam transformer is a reflecting surface, preferably a 45 degree conical reflector, placed in the near field from the electrostatic beam generator and having a geometric surface in which all points of the surface are generated by the revolution of a straight line about a fixed axis which is normal to the plane of said electrostatic transducer. A partial cone surface has proved to be ideal for beam expansion purposes. In a modification, accoustic absorbing material contiguously bounds the conical surface to better define and control the transmitted beam.
Therefore, the basic object of the invention is to provide an improved, low cost accoustic transducer for ranging, guidance and surveillance system.